Compare two columns and produce a list of 'missing' items

It would be very helpful if you posted the structure of the CustID.
Samples of the CustID data would also help.

Are they all numeric? Alphanumeric?
Do they contain dashes (-) or slashes (/)?
Are they duplicated in the list or are they all unique?
Are there column headings for the separate lists?
Are there any blank rows in the lists?

This isn't the typical formula approach, but this is
the way I would typically do what you're asking.....
(It may seem like many steps, but once you see how it works,
you can set the whole thing up in under a minute)

• Name the data ranges in each workbook and save the workbooks.
I called the ranges: rngCustDataBk1 and rngCustDataBk2, respectively.

• Open Book1

From the Excel Main Menu:
<data><get external data><new database query>
Navigate to Book1
...select rngCustDataBk1
...Click the [Next] buttons, then [Finish]

MS Query will be open and you'll see your Book1 data.

• Click the Add Tables button.
• Navigate to Book2 and select the rngCustDataBk2 range.
(it will then display in the MS Query window next to the Book1 data)

• Drag the Customer ID field from Book1 on top of the Customer ID field from Book2.
(a JOIN line will then connect those fields)

• Double-click that line to view the Joins options window
• Select option 2: All values from rngCustDataBk1 (etc)
• Click [Add]....Click [Close]

Almost done....
If the Criteria section is not visible, click the Show/Hide criteria button
• Criteria Field: rngCustDataBk2.Customer ID
• Value: type this--> IS NULL
• Click the Query Now button (or...<records><query now>

The end result displays ONLY Book1 Customer ID's with no match in Book2

If everything looks right, click the Return Data button and
select where you want the results to display.

If you're interested in that approach,
post back if you have more questions.